1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an active matrix (AM) device, and an AM device containing the composition.
2. Related Art
In a liquid crystal display device, classification is based on an operating mode of the liquid crystals, including phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment (VA), and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insular metal (MIM) and so forth. TFT is further classified into amorphous silicon, polycrystal silicon, and continuous grain silicon. The polycrystal silicon is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source includes a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
Table 1 below summarizes a relationship between the general characteristics of liquid crystal compositions and AM devices. The general characteristics of the composition will be explained further based on a commercially available AM device. A temperature range of a nematic phase relates to the temperature range in which the device can be used. A desirable maximum temperature of the nematic phase is approximately 70° C. or more and a desirable minimum temperature is approximately −20° C. or less. The viscosity of the composition relates to the response time of the device. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
General characteristicsGeneral characteristicsNo.of a compositionof an AM Device1Temperature range of aUsable temperature rangenematic phase is wideis wide2Viscosity is small1Response time is short3Optical anisotropy isContrast ratio is largesuitable4Dielectric anisotropyDriving voltage is lowis positively orand electric powernegatively largeconsumption is small5Specific resistanceVoltage holding ratio isis largelarge and a contrastratio is large1A liquid crystal composition can be injected into a cell in a short time.Table 1. General Characteristics of a Liquid Crystal Composition and an AM Device
The optical anisotropy of the composition relates to the contrast ratio of the device. A device having a mode such as VA, IPS and so forth utilizes electrically controlled birefringence. Accordingly, a product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed to be a certain value to maximize the contrast ratio in the VA mode. Examples of the value include approximately 0.30 μm to approximately 0.35 μm (VA mode) or approximately 0.20 μm to approximately 0.30 μm (IPS mode). The cell gap (d) is generally from approximately 3 μm to approximately 6 μm, and the optical anisotropy of the composition is in the range of approximately 0.05 to approximately 0.11. A large dielectric anisotropy of the composition contributes to a small driving voltage. Accordingly, a large dielectric anisotropy is desirable. In an ordinary AM device, a composition having a positive dielectric anisotropy is employed. In an AM device having a VA mode, a composition having a negative dielectric anisotropy is employed. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance in the initial stage is desirable. A composition having a large specific resistance after using for a long time is desirable.
A composition having a negative dielectric anisotropy contains a compound having a negative anisotropy. Examples of the compound include a compound having a negative dielectric anisotropy containing 6-methyl-2,3-difluoro-1,4-phenylene as disclosed, for example, in JP H10-291945 A/1998.